In conventional mixing machines of this type, the rotor and the casing are generally attached to the lower end of a rotating shaft, which is inserted into the mixing tank from the top, and the rotor in the casing is held at an intermediate position in the tank and driven for mixing operation.
In the aforementioned conventional mixing means, the rotor in the casing is driven at a fixed intermediate position in the tank, so that it is not possible to appropriately adapt the mixing operation to a change in the volume of the contents in the tank to be processed. Especially when a large difference between the volume of the tank contents in the initial stage and a subsequent stage of the mixing process arises, it is not possible with the conventional mixers to provide an optimum mixing operation adapted for the particular volume of the contents at each stage. This inevitably results in uneven quality of the product and also a longer processing time. Further, when the powdered material is fed from the top of the tank into the liquid contents therein, it is desired that the mixing operation be performed near the liquid surface because the powdered material tends to float there. However, the conventional mixing equipment is unable to generate agitations that approach the liquid surface to provide an effective mixing operation. In addition, particles of some powdered materials tend to settle on the bottom of the tank due to gravity, in which case the conventional mixing equipment often fails to generate agitations effective enough to disperse the settled particles. Also when liquid contents with different specific gravities are to be mixed, the upper and lower parts in the tank are not uniformly mixed so that uniform emulsion cannot be obtained.